Shaft furnaces, especially blast furnaces, are charged via a lock which is alternatively put in communication with the atmosphere during its filling and with the interior of the furnace during the flow-off of the charging material. For this purpose, the flow-off orifice and the charging orifice of the lock are equipped with sealing shutters. A sealing shutter typically includes a soft peripheral gasket and is movable between a first position, wherein the gasket is laid against an annular seat surrounding the charging orifice, and a second position wherein the gasket is set apart from the annular seat. A flow-off orifice sealing shutter is necessarily exposed to the high temperatures prevailing inside the furnace. These high temperatures quickly subject the gasket of such a shutter to fatigue, particularly because these gaskets are made of a soft material which is relatively vulnerable to mechanical wear and which consequently has to be replaced frequently.
Moreover, when, depending on the operation of the furnace, the temperature in the chamber is low and the seat is cooled below the condensation point of the hot wet gases, the moisture of these condenses on the seat, thus causing the deposit of dust which forms a crust there when the temperature rises again. These crusts reduce the sealing effect and accelerate the wear of the gaskets. U.S. Pat. No. 4,216,946, the disclosure of which is incorporated herein by reference, has proposed reducing this risk of the formation of a crust on the seat by heating it by means of an electrical resistor. Nevertheless, this measure does not make it possible to solve the problem of the overheating of the gasket and also does not protect it in its open position.